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| Click on image to enlarge. Photo © Daniel R. Snyder |
Granular lherzolite xenolith in basalt. Green masses are peridotite (lherzolite), gray mass is basalt. San Carlos Indian Reservation, Arizona. Scale in centimeters.
You probably know that peridotite is the commonest rock on the planet. Starting at the base of the crust and going straight down for 400 kilometers, the Earth's mantle is mostly peridotite as we know it at Earth-surface conditions. Recall from petrology that there is compelling evidence for a phase change at the 400-km level. Because of the very high pressure at that that depth, olivine is thought to convert to a spinel structure - it's still olivine, (Mg,Fe)
2SiO
4, but it has a different crystal structure, and it's about 9 percent denser. This high-pressure polymorph of olivine undergoes another phase change at a depth of 670 kilometers. If that's all the peridotite there was, it would still represent 20 percent of the volume of Earth. Beyond the 670-km. level, and down to 2900 kilometers, everything is still mantle, and is believed to consist of other phase transformations of olivine, as well as pyroxenes and garnet, as pressure increases. So all in all the mantle, consisting mainly of peridotites (variously defined) accounts for 80 percent of the volume of our planet.
If you'd like to learn more about the composition and petrology of the mantle, most college petrology books devote a few pages to the subject. Beyond that, I recommend a very thorough book by A. E. Ringwood, (1975),
The Composition and Petrology of the Earth's Mantle, 604 p., McGraw-Hill. If your library doesn't have it, you can get it used online for about $25.00. Ringwood later (1986) also wrote a shorter review of the subject in the
Proceedings of the 4th International Kimberlite Conference, published by the Geological Society of Australia. It's only 29 pages and contains some updated references, and is an easier (and faster) read than the 1975 tome.
So, of there's so much peridotite on earth, why does so little of it crop out at the surface? As your petrology book will tell you, the short answer to that is that the rocks that form the upper crust are generated by partial melting of peridotite in the lower crust and upper mantle, which forms magma that works its way upward and solidifies as granites and other intrusive igneous rocks if it cools before it reaches the surface, or flows out on the surface as lavas that cool to form basalts and other extrusive igneous rocks. Since this is only
partial melting, what remains behind in the mantle is still peridotite.
How do we know all this if we can't see into, or drill down or sink mineshafts into the mantle? A lot of what we know comes from indirect evidence - mainly seismic wave records, from which rock densities can be inferred. And in the past 50 years, great advances in experimental petrology have allowed scientists to replicate the behavior of rocks and minerals at mantle-like pressures. But in terms of looking at actual mantle-derived peridotites, there are only three ways this can be done. And again, these are listed in most petrology books:
1. Ultramafic xenoliths are carried to the surface by magma rising rapidly through conduits in mantle wall rock. If a basaltic magma moves through peridotite country rock, it will pluck peridotite blocks from the wall of the conduit and carry them to the surface. Since the peridotite melts at a relatively high temperature, while the basaltic magma stays fluid at a lower temperature, the xenolith can reach the surface without being melted by the magma. The image above shows such a xenolith brought to the surface in an alkaline basalt. The deepest-origin xenoliths, from depths of more than 300 kilometers, are brought up in the magmas of kimberlites.
2. Because the oceanic crust is considerably thinner than the continental crust, mantle rocks are sometimes exposed on the ocean floor along fault zones. Of course, these are samples only of the uppermost mantle.
3. Ophiolites are layered mafic and ultramafic rock sequences, formerly oceanic crust and upper mantle, that were emplaced onto active continental margins. Sometimes transported far inland by low-angle thrust faulting, as in the Appalachians, the peridotite component of such an ophiolite survives as a coherent body of ultramafic rocks. The disadvantages of such ultramafic bodies are that they represent a sampling of only the shallowest part of the mantle, they are typically metamorphosed (recrystallized), so the original fabric and structure are destroyed, and they are highly susceptible to alteration into serpentine, talc, chlorite, and other hydrous secondary minerals. The advantage of these bodies - originally, and still, called Alpine peridotites - is that they are widely distributed throughout the world, and are generally accessible for study. Many of the images in the later pages of this blog are from such exposures.
